1. Field of the Invention
The present invention relates to an electrical resistor having a negative temperature coefficient for incremental resistance values, and in particular to a lead structure for such a resistor and a method for the manufacture thereof.
2. Description of the Prior Art
Electrical resistors are known in the art having a negative temperature coefficient for incremental resistance values, generally referred to as a hot carrier, and NTC resistor or an NTC thermistor. Such resistors generally have a wafer of densely sintered metal oxide ceramic having a small diameter, such as in the range of 1.5 mm through 5 mm, and a low thickness, such as 0.6 mm through 2.5 mm. Firmly adhering and solderable coatings are applied to the opposite major faces of the wafer, and current lead elements are secured thereto with solder.
Wafer NTC resistors for temperatures from -60.degree. C. through +300.degree. C., preferably in the range of from -30.degree.0 C. through +150.degree. C. are manufactured, for example, from oxides of the transition metals manganese, iron, cobalt, copper, nickel and zinc by pressing under high pressure and subsequent sintering, as described in Siemens Heissleiter-Lieferprogramm 1984/85 July 1984 at page 2. The ceramic bodies which thereby result are not in a form capable of accepting soldered connections.
Electrical contacting for such a ceramic wafer is therefore generally undertaken by applying firming adhering metal coatings to the major opposite faces of the wafer. Such methods are well known in the art, as described in German OS No. 1 947 799, U.S. Pat. No. 3,676,211 and U.S. Pat. No. 3, 793,604.
When current lead elements are to be soldered to these coatings, the coatings are generally formed of two or more layers of different metals, of which the first layer, adjacent to the ceramic, consists of a metal which forms a firmly adhering ohmic contact with the ceramic material, to the extent such a contact is required, and the outer layer consist of an easily solderable, more precious metal.
For example, German OS No. 2 838 508 describes a method for solderable contact coating of ceramic PTC resistors free of a barrier layer wherein the first layer, consisting of aluminum or of an alloy predominantly containing aluminum, is produced in a silkscreen printing method, and the second layer, consisting of copper, is applied by a Schoop process.
Thermal sensors of preferably small wafer-shaped NTC resistor elements having solderable coatings and leads soldered thereto carrying the resistor element are manufactured in many forms, because an advantage of such a structure is the simply and inexpensive manner by which high numbers of such elements can be produced in an automatic manner.
The aforementioned U.S. Pat. No. 3,676,211 describes a method wherein a ceramic wafers provided with solderable coatings are clamped between crossed leads which are disposed parallel and radially on the electrode surface, the element with the leads being dip-soldered in an automated method and be subsequently provided with an insulating coating as needed.
The method described in U.S. Pat. No. 3,793,604 is also for manufacturing wafer thermistors, wherein the path on which the lead is soldered to the coating does not extend to the edge of the ceramic wafer, but rather extends only over half of the wafer diameter in the middle of the wafer. This structure is intended to effect an improved adhesion of the soldered leads.
When such thermistors having radially soldered leads are exposed to rapidly changing temperatures, thereby causing a temperature shock stress, the soldered connections are subjected to large mechanical loads resulting from the different thermal expansion properties of the ceramic and of the lead.
After repeated temperature shock stresses, the resistance drift of NTC resistor wafers having soldered current lead elements may amount to 100% of the initial resistance.